1. Field of the Invention
The present invention is directed to an electronic ballast for hot cathode discharge lamp having filaments of the type which requires preheating at the start of lighting, and more particularly to the electronic ballast including a chopper and inverter combination for producing a driving voltage to the discharge lamp from a DC power source.
2. Description of the Prior Art
Electronic ballast are widely utilized for driving discharge lamps such as fluorescent lamps. As disclosed in U.S. Pat. Nos. 5,144,195, 4,959,591, and 5,177,408, the electronic ballast is designed generally to comprises a chopper providing from a DC voltage supply a smoothed step-up output from a DC supply and an inverter which is energized by the chopper output to provide a high frequency AC voltage to be applied to the discharge lamp for lighting thereof.
U.S. Pat. No. 5,144,195 discloses a ballast circuit in which the chopper is controlled to start operating in a delayed manner from the start-up of the inverter so that the inverter starts operating in a initial transient period immediately after the energization of the ballast circuit while the chopper is not active enough to provide substantially a steady-state chopper output.
U.S. Pat. No. 4,959,591 discloses a ballast circuit in which the chopper and the inverter are caused to start operating simultaneously after the energization of the ballast circuit such that the inverter output becomes stable after an initial transient period in which the chopper increases its output toward a steady state level.
In the ballast circuits of the above two patents, the inverter acts in the transient period to produce a limited output which may be utilized to preheating the discharge lamp of hot cathode type. However, the inverter suffers at the start of its operation from unstable input voltage and therefore is very likely to cause uncontrolled conditions resulting in abnormal oscillation of the inverter output, excessive noises, or undue stress applied to switching elements in the inverter, all of which should be avoided for safe and reliable ballast operation. Particularly, since the ballast circuits include no positive scheme of controlling the inverter to limit its output during the transient period, the inverter is subject directly to unstable voltage from the chopper during that period and is therefore very likely to cause unexpected and undesired oscillation.
U.S. Pat. No. 5,177,408 disclose a ballast for instant-start lamp, i.e., cold cathode discharge lamp. Although the ballast of the patent is designed to permit the inverter to start operating only after the chopper becomes stable to provide a steady state voltage to eliminate the above problems, the inverter acts to produce its full voltage upon being started and is not controlled to produce a limited voltage as is required for preheating the hot cathode discharge lamp.